How to Tell When Your Carbon Monoxide Alarm Needs Replacing

The short answer: carbon monoxide detectors do expire

Yes—carbon monoxide detectors go bad. Their sensors have a finite service life, so the complete alarm eventually must be replaced. Sensor sensitivity can decline over time, even when the unit still has power (Consumer Reports’ smoke and carbon monoxide detector buying guide).

Battery life and sensor life are separate:

• The battery powers the alarm.
• The sensor detects carbon monoxide.
• Replacing the battery does not renew an aging or expired sensor.

That distinction applies to replaceable-battery, sealed-battery, plug-in, hardwired, and combination alarms. A fresh battery may correct a low-power warning in a model with replaceable batteries, but it cannot make an expired detector new again.

Do not assume an alarm remains within its service life merely because it is powered, quiet, or responsive when you press the test button. Those observations are useful, but they do not override a printed replacement date. A button test checks certain alarm functions; it does not authorize keeping the unit beyond the service period specified by its manufacturer.

Likewise, replace an alarm that its label, display, or manual identifies as being in end-of-life mode. Some units permit temporary silencing of the warning, but silencing it does not repair the sensor or extend the alarm’s rated life.

The controlling rule is simple: follow the replace-by date, label, and instructions for your exact model. General lifespan estimates can help when investigating an old or undated unit, but model-specific information takes precedence.

Rule out a carbon monoxide emergency before troubleshooting

Safety warning: If the alarm may be detecting carbon monoxide—or if you cannot confidently identify the sound as a maintenance signal—move everyone to fresh air and call emergency services from outside. Do not remain indoors to change batteries, reset the alarm, clean it, or look for a date. A continuous alarm, especially when anyone has headache, dizziness, vomiting, or flu-like symptoms, warrants immediate emergency action (Seattle Fire Department guidance on sounding and expired CO alarms).

A continuous or nonstop alarm must not be dismissed as an expiration warning simply because the detector is old. Intermittent chirping is often associated with a low battery, fault, or end-of-life condition, but alert patterns differ by manufacturer and model. The sound alone may not be enough to rule out danger.

Choose the safer interpretation if:

• You do not know the alarm’s brand or model.
• The alert is louder, longer, or more urgent than its normal maintenance chirp.
• Multiple interconnected alarms are sounding.
• The display shows a CO reading or an unfamiliar warning.
• Anyone has possible exposure symptoms.
• Fuel-burning equipment, a vehicle, or a generator has been operating nearby.
• You cannot confidently decode the sound, light, or display pattern.

This is a conservative decision rule rather than a universal manufacturer code: when the alert is uncertain, deal with the possible emergency before diagnosing the device. Once everyone is outside in fresh air, call emergency services and follow their instructions. Consult the alarm’s label or manual only from a safe location.

How long carbon monoxide detectors last

A broad estimate across the supplied guidance is approximately 5 to 10 years, but no single replacement age applies to every detector.

Published guidance varies:

• Consumer Reports gives a general five-year replacement rule.
• First Alert describes a typical service period of five to seven years.
• The Seattle Fire Department describes CO-alarm life expectancy as around seven years.
• Kidde says its alarms generally last seven to ten years, depending on the model.

First Alert pairs its five-to-seven-year range with instructions to consult the exact manual because replacement timing and warning behavior can vary by model (First Alert’s carbon monoxide alarm guidance). The Seattle Fire Department’s estimate is approximately seven years, not a universal expiration date, and its guidance likewise directs users to follow the manufacturer’s instructions.

Kidde’s model history shows why one generic rule can be misleading. Kidde says its CO alarms manufactured before 2013 have a seven-year life. It says alarms manufactured from 2013 onward generally have a 10-year life, with listed exceptions for models KN-COEG-3, KN-COPE-I, and KN-COPP-3. Those figures are specific to Kidde products and must not be applied to another brand (Kidde’s CO-alarm replacement guidance).

Use this order of authority:

1. An explicit replace-by or expiration date on the alarm
2. The service life stated in the exact model’s manual
3. Manufacturer instructions tied to the model or production date
4. A general five-to-ten-year estimate when reliable model information is unavailable

Replace the alarm when its stated service period ends. Do not wait for it to chirp if the printed replacement date has arrived. End-of-life alerts are a backstop, not permission to extend the rated life.

Avoid treating five, seven, or ten years as universally correct. A ten-year assumption could leave a shorter-lived model in service too long, while automatically discarding every five-year-old unit may conflict with the documented life of another model. Read the alarm and its manual rather than estimating from appearance.

Find the alarm’s age and correct replacement date

Use a methodical inspection rather than relying only on when you remember buying the alarm.

1. Read the front display. Record any date, error code, brand, model name, CO reading, or end-of-life message.
2. Follow the alarm’s instructions before moving it. Do not disconnect a sounding alarm while people remain indoors, and do not improvise with hardwired equipment.
3. Inspect the back label. Look for a replace-by date, expiration date, manufacture date, model number, and serial number.
4. Check the battery compartment if the manual permits it. Some units place identifying information inside or beside the compartment.
5. Review packaging and installation records. Receipts, maintenance logs, property records, or a dated note on the housing may help establish age.
6. Locate the exact manual. Search the manufacturer’s website using the complete model number.
7. Calculate a replacement date only if necessary. If the unit gives a manufacture date but no replace-by date, apply the service life stated for that exact model.

Look first for an explicit replacement or expiration date. It avoids the uncertainty involved in calculating age. If only a manufacture date appears, do not automatically add five, seven, or ten years until you have confirmed the model’s rated life.

The alarm may have been manufactured before it was purchased or installed. Manufacturer guidance therefore commonly directs owners to inspect the date printed on the alarm itself.

If the original manual is missing, use the full model number to find instructions from the manufacturer. Do not use a manual for a similar-looking alarm or assume another brand’s signal patterns and replacement schedule apply.

An older alarm with no readable date, model number, or reliable installation record creates a practical problem: its remaining service life cannot be verified. Replacing it is a prudent precaution, although it is not presented here as a universal legal requirement. Responsibilities in rental or managed properties vary by lease and jurisdiction.

For a new alarm, record:

• Brand and complete model number
• Manufacture date
• Installation date
• Stated service life
• Scheduled replacement date
• Approved battery type, if replaceable

Do not cover vents, instructions, certification marks, or other labels. Keep the same information in an accessible household or property-maintenance record.

What chirps, beeps, lights, and display messages can mean

Important: Beep counts, timing, lights, and display codes are not universal. This table is a diagnostic framework, not a substitute for the label or manual for your exact alarm.

Alert category	What you might observe	What it may mean	Appropriate response
Possible active CO alarm	A loud repeating alarm, emergency voice warning, CO reading, or multiple interconnected alarms sounding	Carbon monoxide may be present	Move everyone to fresh air and call emergency services from outside. Do not troubleshoot indoors.
Low-battery warning	An intermittent chirp, low-battery message, or battery light	A replaceable battery may no longer provide sufficient power	After ruling out an emergency, use the approved battery-replacement procedure for that model.
End-of-life warning	A dedicated chirp sequence, ERR, END, or another replacement message	The alarm has reached its rated service limit	Replace the complete alarm. A battery change does not renew the sensor.
Malfunction or sensor error	An error code, unusual light pattern, unpredictable sound, or failed test	The alarm may have an internal fault, sensor error, power problem, or other operational issue	Consult the exact manual and perform only the checks it authorizes. Replace the alarm if the prescribed steps do not resolve the problem.

Certain Kidde digital alarms use Err or End as end-of-life displays and emit a quick beep about every 30 seconds. Kidde identifies these as replacement signals for the specific digital models covered by its instructions—not as universal CO-alarm behavior (Kidde’s digital alarm end-of-life instructions).

Kidde also describes different behavior for other alarm types:

• On certain non-digital Kidde alarms, a quick beep about every 30 seconds with a flashing red light may initially call for fresh batteries. If the same warning continues after the approved batteries are installed, Kidde identifies the alarm as being at end of life.
• On certain Kidde combination alarms, a chirp about every 30 seconds can indicate either end of life or a sensor error. The interval alone does not establish which condition exists.
• A battery replacement will not correct a confirmed end-of-life or sensor-error condition.

First Alert uses different examples in its cited guidance: four beeps followed by a pause is identified as an emergency CO pattern, one beep per minute as a low-battery warning, and five beeps per minute as an end-of-life warning. These are First Alert examples and cannot safely be generalized to other manufacturers or even assumed to cover every model from the same manufacturer.

Never apply a Kidde pattern automatically to First Alert, or a First Alert pattern to Kidde, ADT, a store brand, or another alarm. Identify the complete model and consult its instructions.

Persistent chirping after an approved battery change can indicate an expired alarm or a fault, but it is not conclusive by itself. Check the display, light behavior, dates, and exact manual. If the pattern remains unknown and could represent CO, leave and call emergency services rather than continuing to experiment indoors.

A safety-first troubleshooting decision tree

Use this sequence when an alarm sounds, chirps, displays an error, or fails a test.

1. Decide whether carbon monoxide could be present

Consider the sound, display, surrounding circumstances, and how occupants feel. If the alarm could be reporting CO—or you cannot confidently exclude that possibility—move everyone to fresh air and call emergency services from outside.

Do not begin by removing the battery. Doing so could disable a warning before you know why it sounded.

2. Identify the exact brand and model

After a possible CO emergency has been addressed, record:

• Brand
• Complete model number
• Sound pattern
• Time between chirps
• Display message or CO reading
• Light color and flashing pattern
• Whether interconnected alarms sounded

Compare those details with the label or manual for that exact unit.

3. Inspect the manufacture and replacement dates

If the alarm is at or beyond its printed replacement date or model-specific service life, replace the complete unit. Do not use a reset, cleaning procedure, or battery change to extend the replacement deadline.

If its date cannot be read and reliable records are unavailable, replacement is generally the safer practical choice when the unit appears old because its remaining service life cannot be verified.

4. Address the battery only as instructed

For an alarm with replaceable batteries, install only the battery type approved for the model. Follow the specified procedure, observe polarity, and close the compartment as directed.

Follow the manufacturer’s instructions for replacing the complete alarm when it reaches end of life.

5. Perform only permitted checks

If the manual authorizes them, follow its procedures for inspecting installation, cleaning accessible areas, resetting the alarm, and retesting it. Do not invent generalized electrical or sensor-repair steps.

For hardwired alarms, limit your actions to those permitted by the manufacturer and applicable local requirements.

6. Replace an expired or unresolved alarm

Replace the complete unit when:

• It has reached its printed replacement date.
• Its model-specific signal is confirmed as an end-of-life warning.
• The manufacturer identifies the displayed error as nonrecoverable.
• It continues to fail after the manual’s permitted checks.
• It remains unresponsive after approved troubleshooting.
• Its age cannot be established and it is reasonably suspected to be old.

Replacing a battery will not stop a genuine end-of-life condition. Some alarms may allow an end-of-life signal to be silenced temporarily; Seattle Fire Department guidance describes units that can silence the signal for 30 days, after which it resumes. The alarm still requires replacement.

Silenced does not mean repaired. A silence function controls the sound temporarily; it does not renew the sensor or restore an expired alarm’s detection capability.

Testing and battery maintenance do not replace age-based replacement

Test each CO alarm monthly in accordance with its instructions. First Alert recommends monthly button testing and replacement when an alarm continues to fail after the approved battery, installation, cleaning, and retesting checks.

Interpret a passing button test narrowly. It confirms that certain alarm functions respond during the test, but it does not override a printed replace-by date or prove that an aging sensor retains full sensitivity.

Think of maintenance as three separate obligations:

1. Power maintenance: Keep replaceable batteries functioning according to the model’s schedule.
2. Operational checks: Test the alarm and investigate unfamiliar warnings.
3. Age-based replacement: Replace the complete unit at the end of its rated service life.

For replaceable-battery alarms, use the battery type and schedule in the manual rather than imposing one timetable on every detector. Other units have sealed batteries intended to last for the alarm’s service period and are replaced as a complete assembly.

Use this recurring checklist:

• Press the test button as instructed.
• Listen for the expected response.
• Inspect the housing for visible damage or contamination.
• Confirm that vents and openings are not covered.
• Make sure the date and model information remain readable.
• Look up any unfamiliar chirp, light, code, or voice message.
• Replace the alarm after an unresolved failed test or confirmed end-of-life warning.

Testing does not reset the replacement clock. Neither does changing the battery, cleaning the exterior, restoring household power, or pressing a silence button.

Replacing the alarm: what applies to each power type

When its CO sensor or complete assembly reaches the manufacturer’s specified end of life, every common alarm configuration requires replacement:

• Replaceable-battery alarms
• Sealed-battery alarms
• Plug-in alarms
• Hardwired alarms
• Smoke-and-CO combination alarms

Continuous household power does not make a CO sensor permanent. Neither does a sealed long-life battery. Guidance covering battery-powered, plug-in, and hardwired detectors explains that all still require complete replacement when their sensors expire (guidance on replacing different CO-alarm types).

Before selecting a replacement, confirm that it is suitable for:

• The existing power source
• The mounting arrangement
• Any wired or wireless interconnection system
• The required alarm type
• The manufacturer’s installation requirements

For hardwired equipment, follow the manufacturer’s replacement procedure and applicable local requirements rather than using a universal do-it-yourself wiring method.

When shopping, check for:

• A reasonably recent manufacture date
• A clearly stated service life
• Compatibility with the intended application
• Recognized certification

Consumer Reports specifically advises buyers to check the manufacture date and look for UL certification.

Also verify what the replacement is designed to detect. A CO-only alarm does not detect smoke or explosive gases such as natural gas, propane, or methane. Detection of more than one hazard requires equipment specifically designed and labeled for those hazards; do not assume every “combination” alarm includes combustible-gas detection.

Retain the new manual and record the model, manufacture date, installation date, and scheduled replacement date. In a rental, dormitory, managed building, or shared property, notify the responsible party as appropriate. Placement rules, electrical requirements, disposal procedures, and landlord or tenant duties vary by jurisdiction, so follow the manufacturer’s instructions and applicable local requirements.

Frequently asked questions

Can a carbon monoxide detector pass its test and still be expired?

Yes. A test-button response confirms certain alarm functions, but it does not override a printed replace-by date or establish that an aging sensor remains fully sensitive. Replace the complete alarm when it reaches its stated service limit, even if the test sound still works.

Why is my carbon monoxide detector still chirping after I changed the battery?

The chirp may indicate end of life, a sensor error, a power or installation issue, or another model-specific condition. First rule out a possible CO event. Then check the brand, complete model number, display, light pattern, dates, and manual.

If the manual confirms an end-of-life warning, replace the entire alarm. A fresh battery supplies power but cannot renew an expired sensor.

Do hardwired and sealed-battery carbon monoxide detectors need replacement?

Yes. Hardwired power and sealed batteries do not make CO sensors permanent. Replace the complete unit when it reaches its model-specific service limit, gives a confirmed end-of-life warning, or remains faulty after the checks authorized by its manual.

For hardwired equipment, follow the manufacturer’s procedure and applicable local electrical requirements.

What should I do if my detector has no readable manufacture or expiration date?

Look for the model number on the back, inside an accessible battery compartment, on the packaging, or in installation and property records. Use the complete model number to locate the manufacturer’s instructions.

If neither the age nor the rated service life can be established and the unit appears old, replacement is a prudent safety precaution. Record the installation and scheduled replacement dates for the new alarm.

Does a carbon monoxide detector also detect smoke or natural gas?

Not necessarily. A CO-only detector does not detect smoke, natural gas, propane, or methane. A smoke-and-CO alarm detects both smoke and CO only if it is specifically designed and labeled to do so. Detecting combustible or explosive gases requires equipment specifically intended and labeled for those gases.

The final rule is straightforward: treat any possible CO event as an emergency first, then check the alarm’s label and exact model instructions. Replace the complete unit if it has reached its printed service limit, gives a confirmed end-of-life warning, or continues to fail after permitted troubleshooting. A new battery can restore power, but it cannot make an expired carbon monoxide sensor new again.